icniuhtli.

Headword: 
icniuhtli.
Principal English Translation: 

friend, companion; brother, sister, sibling; relative; descendant from same ancestors; laborer who pertains to a certain estate (see Molina, Karttunen, Lockhart, and attestations)

Orthographic Variants: 
icnīuhtli, icnihutli, tlalicniuhtli, icniotli, ycniuhtli, icniutli
IPAspelling: 
ikniːwtɬi
Alonso de Molina: 

icniuhtli. amigo.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 33r. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

(I)CNĪUHTLI friend, sibling / amigo (M), su hermano, hermana (T for possessed form) The ‘sibling’ sense extends to ‘cousin’ in the phrase HUEHCA ICNĪUH-TLI, literally ‘distant sibling’ See (I)CNĒLIĀ, (I)CNŌ-TL.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 94.

Lockhart’s Nahuatl as Written: 

nocniuh = he/she is my friend (notice here that in the combination of no + icniuhtli the o is stronger, and the i gives way; the rule is that o displaces a following i but gives way to a or e)
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 2.

(i)cnīuh(tli). friend; almost always possessed. the -uh was originally the sing. possessive prefix; the pl. is -cnīhuan (not -cnīuhhuān).
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 219.

Attestations from sources in English: 

auh in tocnioan catca chicunapoalilhuitl oncaxtolli. Auh in toiaovan catca vmpoalilhuitl = They were our friends for 195 days, and our enemies for 40 days. (Mexico City, sixteenth century)
James Lockhart, We People Here: Nahuatl Accounts of the Conquest of Mexico, Repertorium Columbianum v. 1 (Los Angeles: UCLA Center for Medieval and Renaissance Studies, 1993), 176.

ca icniuh in mexicatl = they are the Mexica's friends
James Lockhart, We People Here: Nahuatl Accounts of the Conquest of Mexico, Repertorium Columbianum v. 1 (Los Angeles: UCLA Center for Medieval and Renaissance Studies, 1993), 94.

nocne = my friend
Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.

nicquetzalhuixtoilpiz in icniuhyotli = I tie up the friendship with quetzal-hummingbird [feathers]
John F. Schwaller, "The Pre-Hispanic Poetics of Sahagún's Psalmodia christiana," in Psalms in the Early Modern World, eds. Linda Phyllis Austern, Kari Boyd McBride, and David L. Orvis (London: Ashgate, 2011), 323.

aҫo can ca in jcnjuh, in jtlajximach totecujo: a ie qujcujz = Wherever there is a friend, an aquaintance, of our lord, will he not take it, seize it? (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 84.

tocnihuan = our friends
Daniel Garrison Brinton, Ancient Nahuatl Poetry: Containing the Nahuatl Text of XXVII Ancient Mexican Poems (1887), 155.

otlasoycniuhtzin = my beloved friend (Metepec, Valley of Toluca, 1717)
In Citlalmachiyotl, The Star Sign: A Colonial Nahua Drama of the Three Kings, eds. Justyna Olko and John Sullivan, translation by Louise M. Burkhart (Warsaw: University of Warsaw, Faculty of "Artes Liberales", 2017), 18, 72.

Attestations from sources in Spanish: 

En los documentos de Tlaxcala se le encuentra con dos sentidos diferentes;
1) En composición con otro sustantivo denota igualdad entre personas y es sinónimo de -po: totlacatcaicnihuan, son descendientes como nosotros.
2) Terrazguero. En este sentido, icniuhtli denota vinculación, pertenencia a: ¿Aquitech tipohuia, ac ticniuh ticatca?, ¿A quién perteneces, de quién eres terrazguero?
También se enuentra tlalicniuhtli (ibid) con el mismo significado.

"sustantivo, 'amigo, compañero'. En algunos dialectos significa 'hermano.'"
Thelma Sullivan, Documentos Tlaxcaltecas del siglo XVI en lengua náhuatl (Mexico: Universidad Nacional Autónoma de México, 1987), 40.

themes: 
Family, Kinship, Descent
Friendship, Affection